In a conventional liquid fuel property detection apparatus (for example, patent documents 1 and 2), a light beam emitted by a light emitting device is irradiated into fuel, the light beam transmitted through the fuel is received by a light receiving device, and the concentration of a specific component contained in the fuel is detected based on the amount of received light beam and the like.
In the first conventional technology according to patent document 1, a sensor having a light source is provided. The light source emits a light beam toward fuel flowing in a measurement cell to be supplied to an engine. The light beam emitted from the light source is separated by a semi-reflective separation plate into a light beam reflected by the semi-reflective separation plate and a light beam transmitted through the semi-reflective separation plate. The reflected light beam passes through the fuel in the measurement cell and enters a photo transistor (light receiving element) after being absorbed and attenuated by the fuel. The transmitted light beam, which passed through the semi-reflective separation plate, arrives at the other photo transistor (light receiving element) after being attenuated when passing through the other cell containing a reference fuel (for example, pure methanol). The property of fuel is determined based on a difference between rates of attenuation of the light beams by these cells.
In the second conventional technology according to patent document 2, two light beams of different wavelengths are irradiated to fuel containing a plurality of predetermined components and the concentration of only a specific component is detected based on the transmittance of each light beam. In the second conventional technology, more specifically, the two light beams irradiated toward fuel containing three components of gasoline, ethanol and water are a first wavelength light beam and a second wavelength light beam. The first wavelength light beam has a large difference between light transmittances relative to gasoline and to the remaining components. The second wavelength light beam has a large difference between light transmittances relative to water and the remaining components. The amount of transmission of each light beam in the fuel is measured. The water concentration in the fuel is detected based on the amount of light transmission of the second wavelength light beam. The ethanol concentration in the fuel is detected based on the amount of transmission of the first wavelength light beam and the detected water concentration.    Patent document 1: JP 59-210345A (U.S. Pat. No. 4,594,968)    Patent document 2: JP 2008-281546A (US 2008/0246955A)
Fuel contains hydrocarbon series components (for example, aroma, olefin, paraffin and the like), which are different in bonding state but similar in molecular configuration with respect to carbon. It is technically valuable from the standpoint of, for example, improving fuel economy and exhaust emission of an internal combustion engine, to individually detect such similar hydrocarbon series components.
According to the first conventional technology, it is possible to detect a component, which absorbs only a specific wavelength. However, if it is desired to individually detect similar hydrocarbon components, the light absorption characteristic of each component interfere one another. For this reason, it is not possible to individually detect the similar hydrocarbon components by using a light beam of only one wavelength.
According to the second conventional technology, it is possible to detect the concentrations of three components, gasoline, ethanol and water, which are largely different in molecular configuration.